1. Field of the Invention
The present invention relates to the forming of parts by powder injection molding techniques (PIM), and more specifically to the preparation of master batches (feedstocks) used for injection or extrusion molding.
More specifically, the present invention relates to a master batch of well-determined composition, particularly adapted to the targeted application, and especially to the thermal debinding of molded parts.
2. Description of Related Art
Injection molding techniques (PIM, or microPIM in the case of ultra-fine powders) are currently used to form various objects. Powder injection molding is a method in several steps which combines molding by injection of plastics and the hardening used in powder metallurgy by any type of sintering. It enables to form metal and ceramic components.
In such a method, the first step comprises obtaining a master batch (or feedstock) adapted to the targeted application. The feedstocks are made of a mixture of organic material (or polymer binding agent) and of inorganic powders (metal or ceramic).
Then, the feedstock can be injected like a thermoplastic. Finally, the part is debound and then sintered.
More specifically, feedstocks made of polymer materials, used as a binding agent, and of metal or ceramic powders, are formed at high temperature and injected into a mold. This results in an injected part made of powder-filled polymer, called “green” part. Such a green part has the same shape as the final part but with larger dimensions.
The polymers which have been used to allow the injection of the material then have to be extracted by debinding. The binding agents are thus extracted, after which the resulting “brown” part is sintered to be hardened and densified while homothetically keeping the shape of the “green” part. This results in ceramic and metal parts.
There exist various types of debinding according to the chemical and/or physical composition of the polymers used: catalytic debinding, thermal debinding, solvent, water, or supercritical CO2 debinding.
The catalytic debinding comprises placing the green parts in an inert and very acid atmosphere (nitric or oxalic acid) in an oven. The polymers used in this case are based on polyacetal (for example, polyoxymethylene). Formulations adapted for such a catalytic debinding are for example described in document U.S. Pat. No. 5,531,958.
Thermal debinding is considered as quite simple from a physical point of view, but quite long (up to 60 hours). Despite the disadvantage of the debinding time, it is widely used for many compositions (for example, in documents U.S. Pat. Nos. 5,254,613, 5,417,756, EP 0 599 285, 4,207,226, or ES2167130).
The debinding with a non-polar solvent enables to extract non-polar polymers such as paraffin, polyethylene wax, and more generally of low molecular mass. The solvents used for example are hexane, xylene, toluene. They may also be a combination of several solvents (for example, hexane, trichloromethane, and ethanol) to further remove stabilizers, compatibilizing agents, stearic acid, and others, which are widely used in polymers.
The debinding is then performed by Soxhlet-type chemical extraction. This method, although it uses harmful chemical products, has the advantage of emitting no gas into the atmosphere and of debinding the parts by vaporization-condensation of the solvent. The latter can thus be used again from one debinding to another. The use of filters retaining the extracted polymers allows a debinding with a solvent specific to each cycle.
A chemical debinding may also be used complementarily with other methods, especially thermal debinding or water debinding methods, which enables to improve the kinetics of the general process.
Water extraction debinding also has the advantage of decreasing the emission of gas into the environment, originating from the thermal degradation caused by incineration, and of decreasing the use of solvents or of acids hazardous for the health. Mixtures based on water-debindable polymers are more and more frequently used, as described in documents U.S. Pat. Nos. 6,008,281, 6,264,863, PT102147, 5,098,942, or WO 2010/058371.
Finally, supercritical CO2 debinding, which is currently being developed, enables to extract polymers from feedstocks. It comprises debinding at low temperature and under a CO2 pressure, rapidly and without creating defects (Chartier et al., J. Am. Ceram. Soc. 1995, 78, 1787-1792).
Once debound, the part called “brown” part is extremely fragile. It is current in the art to only partially debind the part before carrying out the sintering, to be able to handle it between the two steps. In this case, a temperature holding period is added during the sintering to totally debind the part. The sintering is carried out in an oven according to current densification processes used in powder metallurgy, that is, a thermal cycle and an atmosphere specific to each material to be sintered.
One of the main elements for the success of a crack-free part having the desired shape and details is the chemical composition of the binding agent used in the feedstock. The mixture should be sufficiently fluid to be injected and molded, and should have a good mechanical hold to be able to strip off and to handle the green part. It is also important to mix a maximum amount of powder with the binding agent, that is, to have a high filler ratio. The filler ratios vary according to the size of the powder and to its chemical and/or physical properties but generally range between 40 and 75% by volume. On materials which have trouble densifying, it is important to increase to a maximum the powder filler ratio in the feedstock.
Most chemical components used to form feedstocks which debind thermally (or in hybrid fashion: thermally and with a solvent, or thermally and with water) comprise a mixture of one or several polymers with paraffin wax and a dispersing agent.
The stable polymers used are semi-crystalline thermoplastics and generally polyolefines, such as polyethylene (low density, high density, linear low density), polypropylene . . . . However, once mixed with a large amount of powder, the mixture is no longer fluid enough to be injected and molded.
To improve the fluidity of the mixture, paraffin (saturated wax) or other unsaturated waxes may be inserted. The paraffin wax is relatively miscible in the polymer matrix with which it is mixed. The portion which is miscible in the stable polymer enables to fluidify the mixture and the other portion of the wax accumulates at the grain boundaries and enables to lubricate at a macroscopic scale the chamber of the extruder or of the injection press. I. Krupa et al. (European Polymer Journal, 43, 2007, p. 4695-4705) explain that the miscibility of the wax depends on the length of the carbon chain of the wax and also on the selected polyethylene (LLDPE or LDPE). To obtain a feedstock with a high filler ratio while keeping a good fluidity of the feedstock, it is necessary to introduce a high quantity of wax in the mixtures.
Guo et al. (Rare metals, vol 28 No 3, June 2009, p. 261) use formulations mainly comprising paraffin. However, a paraffin ratio which is too high, that is, much greater than its solubility in the other polymers used, results in wax accumulation areas at the grain boundaries, which weakens the green part on ejection.
Other formulations involve a smaller quantity of paraffin together with PEG (Poly-Ethylene Glycol), to debind a portion of the part with water rather than thermally. In this case, the compositions are formed with a large majority of PEG of low molar mass (WO 2010/058371, EP 0587 953), which may however result in a lack of hold of the part.
The addition of PEG, which is a small-chain molecule, provides mobility to the large chains of the LDPE polymer, while maintaining the general cohesion due to the hydrogen bonds, and behaving as a plasticizer in the mixtures. It is only very slightly miscible with aliphatic compounds and provides some elasticity to the mixture.
Further, Borealis, the world leader for polyethylene, has described in document WO 2010/72396 a formulation, for the field of electric cables, of a mixture of LDPE and PEG. A method for mixing with chemically incompatible components (for example, LDPE and PEG, the latter being called water-based additive or water tree retardant), but with a sufficient homogeneity to provide the properties expected for the application, that is, a resistant coating of the cables. Typically, it is advocated to insert a quantity smaller than 50% by weight, preferentially smaller than 30%, advantageously ranging between 1.5% and 20%, or even between 2% and 15% of PEG having a molar mass ranging between 4,000 g/mol and 35,000 g/mol, in LDPE.
Anyway, there is an obvious need to develop new feedstock formulations for injection or extrusion molding, compatible with available debinding processes, and especially thermal debinding.